This invention relates generally to the field of semiconductor die manufacturing, and more particularly, to methods for treating the surface of a semiconductor with ozone to enhance bonding of the back surface of a semiconductor die to a substrate using an adhesive.
One of the last steps in manufacturing semiconductor components is adhering the back surface of a completed semiconductor die to a substrate, which is ultimately encapsulated to form a packaged integrated circuit, or mounted directly onto a circuit board or other receiving component. The substrate may be a lead frame or laminated portion of the circuit board or other surface to which the back surface of the die will be adhered with an adhesive. Most typically, the substrate is a lead frame having a substrate surface to which the adhesive binds, and conductive leads to which bond wires extending to bond pads on the front surface of the semiconductor die will be connected. The conventional steps in adhering a semiconductor die to a substrate according to the prior art are explained with reference to FIG. 1A. Several semiconductor die are fabricated on a single wafer having a front surface with a cover layer (e.g., a passivation layer) deposited over fabricated circuit components and a back surface, which is ordinarily comprised of untreated silicon. The back surface of the wafer is polished 10 on a grinding machine and each of the semiconductor die present on the wafer are separated from each other by a dicing step 12. An adhesive paste is applied 20 to the substrate surface and the back surface of the semiconductor die is then attached 30 to the substrate. The adhesive paste is cured by a heating step 40 to harden the adhesive and adhere the semiconductor die to the substrate, which, when the substrate is a lead frame, occurs prior to connecting wires between the bond pads and the leads.
A variety of processes are used to perform the step 40 of curing the adhesive bond between the semiconductor die and the substrate. FIG. 1B illustrates a batch curing process in which individual wafers are processed through the grinding and adhesive application step until a sufficient number have been accumulated 25 to fill a batch oven. The accumulated wafers are then transferred to a curing oven where they are baked in batch 42 typically, for at least 2 hours with the first hour being at about 110xc2x0 C. and the second hour being at about 165xc2x0 C. Batch curing generally produces consistent adhesion between the semiconductor die and the substrates, however, batch curing has other drawbacks. One problem with batch curing is that hundreds or thousands of semiconductor die are exposed in a waiting room for the time required to accumulate enough wafers to fill the batch oven, which increases the chances of surface contamination and bleed. Another problem is that any error in the batch curing process could ruin the entire batch. Still another problem is that batch curing requires a long cycle time, typically about 6 hours between batches, which reduces the speed of manufacturing throughput.
An alternative to batch curing are the so called xe2x80x9csnap curingxe2x80x9d processes illustrated in FIGS. 1C and 1D, in which steps that are common to those of FIGS. 1A and 1B have provided with same reference numerals. In snap curing, each semiconductor die is attached to the substrate with the adhesive paste and immediately placed on a conveyor belt that transports each piece to a curing station 44 where it is incubated for a short period of about 1 to 5 minutes at temperatures of about 130 to 220xc2x0 C. to rapidly cure the adhesive paste. An alternative method of snap curing sometimes used when the substrate is a lead frame, is depicted in FIG. 1D, where the snap cure station is replaced by a wire bonding machine, which is heated to about 150xc2x0 C. for a time sufficient to attach conductive wires between the bond pads of the semiconductor die and the conductive leads of the lead frame. The advantage of using the wire bond machine to perform the snap curing that the process of curing and wire bonding can be performed using the same apparatus. More generally, the principal benefit of snap curing is increased speed of manufacturing throughput. A potential problem with snap curing, however, is occasional failure of the adhering layer due to lack of adequate curing, which may result in delamination of the semiconductor die from its substrate.
In a semiconductor manufacturing facility the above process are automated using various robotic devices. For example, the wafers are transferred from the grinding-machine to the adhesive application station by being momentarily attached and released by a pick-up tip of a wafer exchange machine. A problem sometimes encountered in semiconductor die manufacturing is unwanted stickiness of the back surface of ground wafers to the pick-up tips. Until the present invention, the cause of this stickiness was not adequately addressed.
There is therefore a need in the art to improve the adherence of semiconductor die to its substrates with an adhesive, especially for snap-curing applications, and to overcome the problem of stickiness of the back surface of ground wafers to pick-up tips.
The invention is based on the discovery that adherence of a completed semiconductor die to a substrate by use of an adhesive is enhanced if the back surface layer, typically comprised of silicon is oxidized to form a surface layer comprised of silicon dioxide prior to application of the adhesive. The silicon dioxide surface layer has the added benefit of not sticking to pick-up tips, which aids automated semiconductor manufacturing processes.
In a preferred practice, the back surface of the semiconductor die is oxidized to form a layer of silicon dioxide by contacting the semiconductor die with ozone. In one practice, the silicon dioxide surface layer is formed by contacting the semiconductor die with a gas mixture containing ozone. In another practice, the semiconductor die is contacted with a liquid solution comprising water and ozone. Contacting the semiconductor die with ozone forms a silicon dioxide layer of at least about 10 angstroms thickness in a time period of less than 10 minutes and typically within about 5 minutes. The silicon dioxide layer is formed by contacting the semiconductor die with ozone at room temperature (about 20xc2x0 C. to about 30xc2x0 C.). The silicon dioxide layer enhances the bonding of the semiconductor die to an adhesive. In other practices, the silicon dioxide layer is formed by any step that includes oxidizing the silicon layer on the back surface of the semiconductor die or depositing a layer of silicon dioxide of at least about 10 angstroms thickness prior to applying the adhesive. Also provided are systems that include a station adapted to receive a ground silicon die into a chamber configured to incubate the die in an environment that causes the formation of the silicon dioxide layer of at east 10 angstroms thickness on the back surface of the die prior to application of the adhesive.
The formation of the silicon dioxide layer on the back surface of the semiconductor die is preferably combined with rapid curing processes which include curing the adhesive for less than 5 minutes, or more typically, for about 10 seconds to about 30 seconds, at temperatures of about 130xc2x0 C. or greater, most typically at about 150xc2x0 C. The curing processes may occur in a wire bonding machine, a solder reflow oven, or in any place suitable for incubating the semiconductor die at a selected temperature.